1. Field of the Invention
This invention relates to the field of semiconductor apparatus, and more particularly to semiconductor charge coupled devices having a split-electrode transversal filter configuration.
Semiconductor charge transfer devices generally are of two basic types: charge coupled devices (CCD) and bucket brigade devices (BBD). Either of these types, preferably the CCD, can be used for transfer of signal charge packets along a semiconductor signal charge transfer channel and can be built in the form of a transversal filter device, that is, a tapped delay line configuration with suitably weighted outputs. Such a filter device contains many stages, typically of the order of ten or more, each stage containing a split-electrode having two electrode segments, of equal widths but different lengths, for sensing the signal charge packet instantaneously in that stage of the channel. Typically, the effective charge sensing lengths (l.sub.1 and l.sub.2) of the two segments of such a split-electrode pair in a given stage are characterized by a ratio, r=l.sub.1 /l.sub.2, in accordance with a predetermined tap weight for that stage; whereas the sum of the lengths (l.sub.1 +l.sub.2) of the two segments of a split-electrode pair is the same for all such split-electrode pairs, corresponding to the width of the underlying semiconductor signal charge transfer channel. The effective tap weight r.sub.i of that stage is then given by: r.sub.i =(l.sub.1 -l.sub.2)/(l.sub.1 +l.sub.2). One electrode segment (l.sub.1) of every split-electrode is conductively coupled to a first common output terminal for the charge transfer device, and the other electrode segment (l.sub.2) of every split-electrode is conductively coupled to a second common output terminal for the charge transfer device. For convenience of description, all the electrode segments that are connected to the first common output terminal are referred to as forming the "first set" of sense electrodes, and all the electrode segments that are connected to the second common output terminal are referred to as forming the "second set" of sense electrodes.
During operation of such a charge transfer device of the split-electrode type, there will be a sequence of periodic time intervals (or time slots) during which everyone of these split-electrode segments is sensitive to the corresponding underlying signal charge packet in the semiconductor signal charge transfer channel by reason of induced electrical image charges, so that signals (S.sub.1 and S.sub.2) are periodically developed at the output terminals respectively of the first and second set of electrodes, each such signal being proportional to the sum of the various charge packets underlying all the various electrodes in that set, with each such packet multiplied by the corresponding tap weight. The desired output signal of the device is then the sequence of instantaneous differences between the signals periodically developed during the aforementioned time slots at the two output terminals; that is, the desired (difference mode) output signal (S.sub.1 -S.sub.2) for a given time slot is proportional to: EQU .SIGMA.Q.sub.i (1+r.sub.i)/2-.SIGMA.Q.sub.i (1-r.sub.i /2=.SIGMA.Q.sub.i r.sub.i ( 1)
where r.sub.i is the effective tap weight of the split-electrode pair of the i'th stage, and Q.sub.i is the charge packet in the i'th stage during the given time slot.
In the use of such a CCD transversal filter as a bandpass device, such as a tone detector, it is desirable to have a threshold level of detection such that if, and only if, the input signal contains significant components in the frequency range of interest will there be any output from the filter. Likewise, in the cases of lowpass and highpass filters, a similar threshold level is desirable. In many applications of filters, it is also desirable that this threshold level be insensitive to variations in processing parameters, as well as voltage and temperature fluctuations. Also, it is often desirable to be able to control (change or "shift") the threshold level by external means. Although such threshold level shifting can obviously be achieved by known techniques for varying the threshold level of amplifier detectors of the output of the CCD filter; nevertheless, such an approach suffers from the problem that the resulting threshold level is not stable with respect to fluctuations in the gain of the CCD transversal filter channel caused by fluctuations in such parameters as the oxide thickness and electrode width.
In U.S. Pat. No. 4,032,867, issued to Engeler et al on June 28, 1977, a split-electrode CCD low-pass transversal filter is disclosed in which an added auxiliary charge transfer channel propagates a constant preselected balancing charge. The auxiliary channel runs along the entire length of filter parallel to the signal charge transfer channel underneath extensions of every sense electrode of the second set, the size of these extensions being selected such that the sum of the effective areas (and hence capacitances) of the first set of sense electrodes is equal to the sum of the effective areas of the second set of sense electrodes plus the extensions thereof. Thereby, the filter is electrically "balanced", that is, the output voltage difference between the first set and the second set of sense electrodes is equal to zero ("quiescent point") when the input to the signal channel is at a predetermined fixed (D.C.) level, depending upon the magnitude of the preselected balancing charge. Although this approach for obtaining threshold detection levels might be generalized to encompass the case of bandpass filters; nevertheless, in operation, unless radically different charge densities are used in the auxiliary charge transfer channel from those in the signal charge transfer channel, the required auxiliary channel width becomes inordinately small for the case of a bandpass filter whose number N of sense electrodes is over a hundred. For example, in the case of N=150, the use of equal charge densities in the auxiliary and signal channels causes the required auxiliary channel width to be less than two micron for a threshold level of 20 dB below the maximum CCD signal, that is, below the minimum geometry of about four microns in present-day technology. On the other hand, the use of radically different charge densities in the auxiliary channel from those in the signal channel ("nonmatched" operation) would result in undesirable level shifting caused by uncorrelated fluctuations of temperatures and control voltages in the auxiliary channel vs. the signal channel. Therefore, it would be desirable to have a controllable threshold level for signal detection in a CCD filter which mitigates these problems.